Is zakat due on wealth acquired through unlawful and/or haram sources?

All unlawful and/or haram wealth is impure, thus it cannot be cleansed. It should be returned to its owner if it is possible to track down who they are. If it is not, then it is compulsory for a person to get rid of all of the unlawful wealth (for example, by giving it away to charity without expecting any reward from Allah) out of his possessions in order to free himself from unlawful wealth and not to give it as zakat.